The present invention relates to a multiple phase modulator and, more particularly, to a 2.sup.N -phase digital signal phase modulator where N denotes an integer equal to or greater than 2 and is equal to the number of digital signals input to the modulator.
The conventional modulator used for digital communication system is usually made of a double-balanced (or "ring") modulator operative up to the VHF band. For a frequency region higher than the VHF band, a modulator using a distributed constant circuit has been generally used. Those modulators are complicated and, therefore, require a large amount of time for adjustments.
For frequency bands lower than VHF band, 2.sup.N -phase digital phase modulators have been put into practical use since integrated digital counter circuits can be used for such low frequency bands. This type of conventional 4-phase phase modulator comprises two circuit arrays arranged in parallel, each array including a 1/2 frequency-divider counter circuit and a 2-phase modulator circuit (e.g. an exclusive OR circuit). The 4-phase phase modulator also includes a combining circuit such as a transformer to combine signals from the two circuit arrays. Likewise, 8-phase phase modulators require at least three counter circuits, three 2-phase modulator circuits and two combining circuits.
These conventional modulators are costly to manufacture because they require a large number of counters and a transformer, which are not suited for fabrication in the form of an integrated circuit. Also, phase errors are unavoidable due to fluctuations in delay caused by the two arrays, thereby deteriorating the characteristics in the combined output.